Electrode materials with the olivine structure LiFePO4 (triphyllite) and the quasi-isomorphous delithiated material □FePO4 (wherein □ refers to an Li vacancy; after Li has been removed from LiFePO4, Fe(2+) shifts to Fe(3+) to maintain charge balance) have the advantage of an operating voltage of 3.5 V vs. Li+/Li°, i.e., in the stability window of both liquid and polymer electrolytes but with a flat charge/discharge (lithium intercalation) plateau. The favourable properties of reversibility and appreciably fast kinetics are believed to be associated with the confinement of both Li and Fe at its two oxidation states in an octahedral environment. The manganese derivative LiMnPO4, which is active at 4.2 V vs. Li+/Li°, is even more ideally suited to work with liquid electrolytes. For both the Fe and Mn derivatives, the absence of miscibility at room temperature between the two phases limits considerably the electronic conductivity through small polarons on the transition metal sites.